Modified polyamic acid from aminophthalic acid anhydride and dianhydride

ABSTRACT

Processes are described for the preparation of substantially pure aminophthalic anhydrides and for polymerizing these monomers to unmodified and modified improved polyamic acid and polyimides.

This is a division of application Ser. No. 576,469 filed May 12, 1975,now U.S. Pat. No. 4,064,113, which in turn is a division of priorapplication Ser. No. 370,286, filed May 12, 1975, now abandoned, but forwhich a continuing application was filed July 25, 1975, Ser. No.598,929, for which Patent No. 3,979,416 was granted on Sept. 7, 1976.

PRIOR ART

A synthesis of 4-aminophthalic anhydride was reported originally in JACS30, 1135 (1908). The properties described for this aminophthalicanhydride cast doubts on its identity. Later, J. Brandt in J. Prakt.Chem., Vol. 7, Ser. 4, 163-172 (1958) established that it had not beensynthesized; this later reference describes inadequately the synthesisof 4-aminophthalic anhydride in low yields by the reduction of4-nitrophthalic anhydride. Brandt also describes the formation of apolyamic acid of aminophthalic anhydride, followed by its isolationunder hydrolyzing conditions, as a low molecular weight product. U.S.Pat. No. 3,450,678 describes a process for preparing film-formingpolyamide-acids from aminophthalic anhydrides by reaction in an inertsolvent; it does not describe the synthesis of the aminophthalicanhydrides.

THE DISCLOSURE

This invention relates to the aminophthalic anhydrides of the formula H₂NC₆ Y₃ (CO)₂ O and to polymers derived therefrom, wherein C₆ representsthe benzene ring to which is attached one NH₂ group, and an anhydridegroup in the 1,2- positions, and Y represents hydrogen or a halogenselected from the class of bromine, fluorine and chlorine. Theseaminophthalic anhydrides can be formulated also as ##STR1## wherein Yrepresents a halogen selected from the class of F, Br and Cl, and n isan integer having a value of zero to three. Illustrative examples ofAMPA are

3-aminophthalic anhydride,

4-fluoro-, 3-aminophthalic anhydride,

5-bromo-, 3-aminophthalic anhydride,

6-chloro-, 3-aminophthalic anhydride,

4-fluoro-, 3-aminophthalic anhydride,

3-fluoro-, 4-aminophthalic anhydride,

5-fluoro-, 4-aminophthalic anhydride,

6-fluoro-, 4-aminophthalic anhydride,

4,5-dichloro-, 3-aminophthalic anhydride,

4,5-dibromo-, 3-aminophthalic anhydride,

4,6-dichloro-, 3-aminophthalic anhydride,

3,5-dichloro-, 4-aminophthalic anhydride,

3,6-dichloro-, 4-aminophthalic anhydride,

5,6-dichloro-, 4-aminophthalic anhydride,

4,5,6-trichloro-, 3-aminophthalic anhydride,

3,5,6-tribromo-, 4-aminophthalic anhydride,

3,5,6-trichloro-, 4-aminophthalic anhydride,

3,5,6-trifluoro-, 4-aminophthalic anhydride,

3-chloro-, 5-bromo-, 6-fluoro-, 4-aminophthalic anhydride, etc.

One objective of the present invention is a method of preparingsubstantially pure aminophthalic anhydrides in relatively high yields bythe catalytic reduction of the corresponding precursor nitro compounds.Another objective is to prepare polymers directly by the reduction ofthe nitro compounds, O₂ NC₆ Y₃ (CO)₂ O, in solution. A still furtherobjective is to prepare modified, improved novel polymers of theaminophthalic anhydrides by a process which comprises modifying thepolymerization of said anhydride by coreactive compounds selected fromthe class of aromatic di-primary amines, Ar'(NH₂)₂, and aromaticdicarboxylic anhydrides, Ar[(CO)₂ O]₂, both of which are more fullydescribed hereinafter. It is a further purpose of the invention toprepare the modified polymers by coreaction of the diamine ordianhydride modifiers with the isolated aminophthalic anhydrides orduring the course of the catalytic reduction of the precursornitrophthalic anhydrides. Still another objective of this invention isto prepare modified polymers having either terminal amino, --NH₂ groupsor dianhydride, >(CO)₂ O, groups which have greater utility than theunmodified polymers of the aminophthalic anhydrides. Other objectiveswill become apparent as the description of the invention proceeds.

I have discovered that the pure aminophthalic anhydrides, AMPA, can beprepared by catalytically hydrogenating the corresponding nitrophthalicanhydrides, O₂ NC₆ H₃ (CO)₂ O, in a non-reactive organic solvent at aconcentration of no higher than about 30% by weight of anhydride at atemperature in the range of about 0° C to about 27° C, and thereafterrecovering the aminophthalic anhydride by precipitation. I haveestablished that evaporation of the solvent is an unsatisfactory methodof isolation, if high yields of high purity AMPA are to be achieved.Also, I have found that when organic compounds are used as precipitantsfor AMPA in the organic solvent used in the synthesis, the yields arevery low, as for example, when hexane or benzene are added to solutionsof AMPA in dioxane or dimethylacetamide respectively.

In the process of this invention, a non-reactive organic solvent whichis water-soluble is the preferred solvent since precipitation of theaminophthalic anhydride occurs by the addition of the solution to waterrefrigerated to a temperature of about -30° C to about 20° C, preferablyfrom about 0° C to about 5° C, without hydrolysis of the anhydride.Below 0° C, the precipitant-water obviously is in the form of ice, whichpreferably is crushed or in the form of snow. At higher than about 20°C, the risk of the hydrolysis of the anhydride to the diacid is high.The precipitated AMPA is readily removed by filtration or bycentrifugation and dried in vacuo without heat. By this process, I haveobtained the pure aminophthalic anhydrides in yields in excess of 90%and approaching theoretical values, and in high purity. For example, Ihave prepared the previously undisclosed substantially pure4-aminophthalic anhydride, of a melting point of 207°-208° C, andcharacterized it by elemental analysis, infrared spectroscopy and byformation of known derivatives.

The ratio of the volume of water to the non-reactive water-solubleorganic can be varied over a wide range depending particularly on thenature of the organic solvent and the concentration of the AMPAdissolved therein. Preferably, the amount of water is determinedexperimentally and selected to afford the maximum yield of AMPA,preferably a yield above 95% of AMPA. Generally, the amount of waterwill vary from about 1/4 to 5-10 times the volume of the solvent. Apreferred operating range is from about 1 to 5 volumes of water pervolume of solvent.

In contrast to this process, I have observed that if the catalytichydrogenation is performed in an inert organic solvent, and isolation ofthe AMPA is attempted by evaporation of the solvent, even without heat,part or all of the AMPA is converted to an oligomer or polymer, and, ifany monomer is isolable after concentration, the yield or the purity orboth are low. I have also discovered that if the concentration of thenitrophthalic is higher than about 30% by weight in the non-reactivesolvent, or if the temperature exceeds about 25° C by about 5°-10° C,the yield of AMPA is substantially reduced. Typical suitablewater-soluble organic solvents for hydrogenation of the nitrophthalicanhydrides from which AMPA can be isolated by precipitation with waterare N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF),N,N-diethylformamide, N,N-dimethylmethoxyacetamide,N-methyl-caprolactam, N,N-diethylacetamide, dimethyl sulfoxide,N-methyl-2-pyrrolidone, tetramethylurea, pyridine, dimethylsulfone,hexamethylphosphoramide, formamide, N-methylformamide, butyrolactone,N-formylpiperidine, N-formyl-pyrrolidone, dioxane, tetrahydrofurane,etc., or mixtures of such solvents with each other or with minor amountsof solvents having limited solubility in water, such as nitromethane,tetramethylenesulfone, etc.

I have further discovered that I can prepare a polyamic acid of AMPAhaving an inherent viscosity of at least 0.30 dl/g as 0.5% solution inDMAC at 30° C directly in solution by a preferred modification of theprocess of this invention which comprises catalytically hydrogenating at30°-100° C a 30-70% solution of AMPA in an organic solvent in which thepolymer is also soluble. When the temperature of hydrogenation is aboveabout 35° C, preferably an insoluble, inorganic dehydrating agent ispresent in sufficient quantity to inactivate the water formed in thereduction of the --NO₂ group to an --NH₂ or water formed by thecyclization of some of the hemiamic structures, which water is otherwisecapable of hydrolyzing the amide linkages in the chain and causing amarked reduction in inherent viscosity. The hydrolytic effect of waterincreases with temperature and the presence of dehydrating agent in thesystem is more critical than at lower temperatures, at which it can beused to assure high intrinsic viscosity. For these same reasons,dehydrating agents may be used, if desired, when the process of theinvention is used to prepare monomeric AMPA. Usually, I prefer tooperate at temperatures at which the inorganic dehydrating agent is notrequired critically. Typical inorganic dehydrating agents are, magnesiumsulfate, sodium sulfate, calcium sulfate, barium sulfate, alumina,molecular sieves, etc.

I have also discovered that I can produce modified polymers of AMPAduring the course of the hydrogenation of the nitrophthalic anhydrideunder the conditions described hereinabove, when the reduction isperformed in the presence of the coreactive compounds, O(OC)₂ Ar(CO)₂ Oand H₂ NAr'NH₂ such as are used to modify the polymerization ofpreformed monomeric AMPA.

As coreactive anhydride modifiers to be used in the practice of thisinvention, any organic aromatic-type compound is suitable which has twodicarboxylic acid anhydride groups in which the CO moieties in theanhydride groups are ortho-disposed to each other, O(OC)₂ Ar(CO)₂ O, theonly requirement in these compounds being that the anhydride groups areattached directly to the aromatic ring. This Ar can be the tetravalentnucleus from a single aromatic ring such as benzene, toluene, xylene,etc., or fused aromatic rings such as naphthalene, anthracene,phenanthrene, etc., or aromatic-heterocyclics such as pyridine,quinoline, quinoxaline, etc., as well as a multiplicity of such aromaticnuclei linked together directly to each other or by means of --O--,--S--. --CO--, --SO₂ --, --CR₂ --, ##STR2## etc., linkages in which Rrepresents hydrogen or a hydrocarbon group containing one to twelvecarbon atoms such as --CH₃, --C₆ H₁₂, --C₆ H₅, --C₁₂ H₂₅, or by esterlinkages such as by --COO--, or by imide linkages such as ##STR3## and---CONR--, or by quinoxaline linkages, etc. In each case both >(CO)₂ Ogroups are attached to six-membered aromatic rings, such as for example,to the structures: ##STR4##

The substituents not occupied in the structures by >(CO)₂ O groups areoccupied by hydrogen or any of the substituents which normally are foundon aromatic-type structures such as halogen, for example, Cl and Br,nitrile, alkoxy, aryloxy, hydrocarbon, etc., with the hydrocarbon andhydrocarbonoxy portions having no more than 10 carbon atoms, buthydrogen and halogen are preferred.

For reasons of economy and availability, the O(OC)₂ Ar(CO₂ O compoundscan also be oligomeric or polymeric such as are derived by the reactionof n + 1 moles of monomeric dianhydride with n moles of the aromaticdiamine, Ar(NH)₂, having the general formula O(OC)₂ Ar(CO)₂ [NAr'N(OC)₂Ar(CO)₂ ]_(n) O, so that the anhydride groups are termini in thestructure wherein the value of n is at least one and as high as 10 or20. A typical example of such a dianhydride is ##STR5##

Thus, in the case of pyromellitic anhydride, O(OC)₂ C₆ H₂ (CO)₂ O, themolecular weight is of the order of 220 and in case of the anhydrideend-capped oligomer of an n value of 10, the molecular weight is of theorder of 6200. For reasons of availability and economy, ##STR6## whereinn represents 1 to 10, are preferred compounds for use in the practice ofthis invention. The synthesis of such anhydride-terminated oligomers isillustrated below in examples 27-30.

In the diamine compounds, Ar'(NH₂)₂, used as modifiers in the practiceof this invention, Ar' is the divalent moiety corresponding to Ar inO(OC)₂ Ar(CO)₂ O except that two additional valencies in Ar are occupiedby hydrogen, R, halogen, nitrile, and OR, but hydrogen and halogen arepreferred. Some representative diamines are ##STR7##

The Ar'(NH₂)₂ compounds can also be oligomeric or polymeric such as arederived by the reaction of (n+1) moles of aromatic diamine and n molesof O(OC)₂ Ar(CO)₂ O having the general formula ##STR8## so that the NH₂groups are terminal in the structure wherein n has a numerical value ofat least one and as high as ten or twenty. A typical example of such adiamine is ##STR9## The synthesis of such arylamino-terminated oligomersis illustrated below in examples 23-26.

In the case of C₆ H₄ (NH₂)₂ the molecular weight is of the order of 108and that of the above diamino end-capped oligomer in which n=10, is ofthe order of 6200.

Thus, the amount of the dianhydride or diamine modifier used in thepractice of this invention may be varied over a wide range ofconcentration, due to the wide range in molecular weights of thesemodifiers. For example, when the molecular weight of the modifier islow, of the order of 100 to 300 or 500, the molar ratio of thesemodifiers to AMPA may be in the range of 1 to 100 to 1 to 1000, and whenthe molecular weight is in the range of 2500, the mole ratio of themodifier to AMPA may be as low as 1 to 10 and up to 1 to 1000; and whenthe molecular weight is in the range of 5000, the mole ratio may be inthe range of 1 to 5 to 1 to 5000.

In the embodiment of this invention in which the nitrophthalic anhydrideis reduced to AMPA and polymerized in the course of the reduction, thesolvent used is an organic solvent in which the polymer as well as themonomer is substantially soluble. This requirement is met substantiallyby the list of solvents given heretofore hereinabove fromN,N-dimethylacetamide to N-formylpyrrolidone, inclusive, as well as by

pyridine,

tetramethylenesulfoxide,

pentamethylenesulfone,

N,n-bis(cyanomethyl)formamide,

N,n'-diformyl-piperazine,

N,n-dimethylcyanamide,

m-cresol,

xylenol,

N,n-dimethylaniline,

N,n,n',n'-tetramethyl-alpha-ethylmalonamide,

N,n,n',n'-tetramethylglutaramide,

N,n,n',n'-tetramethylsuccinamide,

thiobis-(N,N-dimethylacetamide),

bis(N,N-dimethylcarbamylmethyl) ether,

N,n,n',n'-tetramethylfumaramide,

methylsuccinonitrile,

1,2,3-tricyanopropane,

alpha-ethylsuccinonitrile,

succinonitrile,

N,n-dimethylcyanoacetamide,

N,n-dimethyl-beta-cyano-propionamide,

dimethylester of methane disulfonic acid,

diethylester of ethane-1,2-disulfonic acid,

bis-(cyanomethyl)-sulfone,

1,2-diethiocyanopropane,

bis-(thiocyanomethyl) ether,

beta-thiocyanoisobutyronitrile,

malonitrile,

N-acetyl-2-pyrrolidone, etc.

The solvents can be used alone, in combination of solvents, or incombination with poorer solvents such as ketones, such as methyl ethylketone; nitroalkanes such as nitroethane, nitropropane, etc., ornon-solvents such as minor amounts of benzene, benzonitrile, dioxane,xylene, toluene, trimethyl benzene and cyclohexane, which can also actas azeotroping agents to remove water of condensation.

The following examples illustrate the practice of this invention. Allpercentages, here as well as throughout the specification, are by weightunless otherwise specified.

EXAMPLE 1

In a 100-ml round-bottom flask equipped with an inlet tube fordeoxygenated nitrogen, a gas outlet tube, a thermometer extending to thebottom of the flask and an electric heating mantle, was placed 21.1 g of4-nitrophthalic acid and the apparatus was flushed with nitrogen. Themass was heated under a slow flow of nitrogen for 4 hours at 165°-170°C, and then allowed to cool to room temperature. The crude product wasadded to toluene, heated to reflux, and filtered while hot. The filtratewas cooled to 0° C, the solid precipitate recovered by filtration,recrystallized from chloroform and dried to constant weight at 35° C ina vacuum oven, to yield 11.08 g of 4-nitrophthalic anhydride, m.p.123°-124° C, whose percent elemental analysis of C, 49.81; H, 1.53; N,7.22 is in excellent agreement with the theoretical values.

EXAMPLE 2

A mixture of 21.1 g of 4-nitrophthalic acid and 50.0 g of aceticanhydride were heated at reflux for 6 hours, following which the aceticacid was removed by distillation at atmospheric pressure. The excessacetic anhydride was removed by distillation at 15 mm Hg pressure withthe flask temperature not exceeding 120° C, leaving a quantitative yieldof crude 4-nitrophthalic anhydride, m.p. 114°-116° C. The product wasrecrystallized from chloroform to afford a 93% yield of product, m.p.123°-124° C, identical to that of Example 1.

EXAMPLE 3

3-Nitrophthalic anhydride was prepared from 3-nitrophthalic acid andacetic anhydride by the same procedure given for its 4-nitro isomer inExample 2. The yield of recrystallized product was 17.85 g (92.5%); m.p.163°-164° C, whose percent elemental analysis of C, 49.79; H, 1.57; N,7.22 is in excellent agreement with the theoretical values.

EXAMPLE 4

Phthalic anhydride was nitrated by the procedure described by Miller inAnn. 208, 223 (1881) and by Bogert in JACS, 28, 617 (1906) to afford anapproximately 50:50% mixture of 3- and 4-nitrophthalic acid which wastreated with acetic anhydride by the procedure of Example 2, resultingin a 91.6% yield of the recrystallized mixture of anhydrides, having anm.p. range of 102°-106° C. The percent elemental analysis of C, 49.81;H, 1.57; N, 7.22 is in excellent agreement with the theoretical valuesof C, 49.74; H, 1.55; N, 7.25.

EXAMPLE 5

In this example, 4-nitrophthalic anhydride was reduced by the proceduregiven by Brandt in J. Prakt. Chem., Vol. 7, Ser. 4, 167 (1958). Withinthe reaction chamber of a hydrogenation apparatus, in 30 ml of dioxane,there was dissolved 3.0 g of 4-nitrophthalic anhydride, to which wasadded 0.5 g of Raney Nickel (W. R. Grace No. 28 Raney Active NickelCatalyst in Water, Type W-2, which was washed first with methanol andthen with dioxane). Then the apparatus was pressured to 250 psi withhydrogen and an exotherm occurred during the first hour of thehydrogenation, which raised the temperature from 24° C to 36° C. Thehydrogenation was terminated at the end of 8 hours. The solution wasfiltered to remove the nickel and divided into three 10-ml portions.

Portion A

This 10-ml portion was processed by Bandt's procedure and was added to100 ml of 0.5 N HCl solution, yielding a flocculent precipitate whichwas isolated by filtration and redissolved in 100 ml of 2% NH₃ solutionand reprecipitated by 0.5 N HCl solution, isolated and dried. There wasobtained 0.47 g of a white solid, which did not melt when heated to 300°C; its equivalent weight by titration with 0.1 N NaOH was 183.3 and itsinfrared spectrum was identical to that of an authentic sample of4-aminophthalic acid of an equivalent weight of 181.1. Its analyticpercent values were C, 53.1; H, 3.79; N, 7.65; it had an inherentviscosity of 0.5 g in 100 ml of dimethylacetamide at 25° C of 0.09 dl/g.

The elemental analysis reported by Brandt (p. 167) of percent C, 53.3;H, 3.8; N, 7.6 for his product as a hydrate of 4-aminophthalic anhydrideis in good agreement with the calculated values of percent C, 53.04; H,3.90; N, 7.73 for 4-aminophthalic acid.

Portion B

The second 10-ml portion was concentrated by evaporation of the dioxaneat 25° C without heating at 10 mm Hg pressure to approximately 3 ml of aviscous mass, after which a sample of approximately 1 ml was spread as athin layer on a glass plate and placed in an oven at 110° C for 10 hoursto eliminate residual dioxane (B.P. 101° C) and there was obtained afilm whose infrared spectrum showed bands for --COOH in the 3000 cm⁻¹region and for --CONH-- in the 1660 cm⁻¹ region.

The remaining material of approximately 2 ml was washed with benzene anddried in a vacuum oven to yield 0.69 g of resin whose inherent viscosityof 0.5 g in 100 g of dimethylacetamide at 25° C was 0.23 dl/g.

Portion C

The third 10-ml portion was isolated by precipitation by adding itslowly with rapid stirring to 150 ml of benzene to yield a yellowprecipitate which was removed by filtration and dried in vacuo at roomtemperature. Its inherent viscosity was 0.24 dl/g and it was soluble indimethylacetamide from which films were cast on glass plates by dryingat 115° C for 24 hours.

Brandt's procedures show that with Raney Nickel catalyst at roomtemperature and a 10% solution of 4-nitrophthalic anhydride in a solventsuch as dioxane, which does not react with anhydride groups, reductionof the amine group occurs which is accompanied by an exotherm inducingthe polycondensation to a hemiamic acid polymer, and films could be castfrom the solutions having the inherent viscosity as prepared.Alternately, the solution of the polymer can be concentrated byevaporation of the dioxane solvent without heating or the polymer can beisolated by precipitation by means of a non-solvent such as benzene.Then the precipitated polymer can be dissolved in an entirely differentsolvent and cast into films. However, if Brandt's solution, whichalready contains the polyamic acid, is treated with aqueous acids andalkalis as a method of isolating the polymer, the polymer is hydrolyzedto lower molecular weights incapable of forming films. Brandt (p. 164)used the diazo reaction of the amino group for the end-groupdetermination on this acid-alkali-water isolated polymer and concludedthat the hemiamic acid polymer contained eight to 10 blocks (units) inits linear chain. Since the diazo reaction is also performed in anaqueous solution with nitrous acid generated from hydrochloric acid andsodium nitrite, further degradation undoubtedly occurred in thedetermination. This sensitivity of amic acid polymers to hydrolysis bywater has been noted in the polyamic acid of pyromellitimides and issimilar to the sensitivity of phthalamic acids previously noted by M. L.Bender in JACS, 79, 1258 (1975).

EXAMPLE 6

In this example, 4-nitrophthalic anhydride was reduced by the proceduregiven by Brandt, cited in Example 5 above.

Three g of 4-nitrophthalic anhydride were dissolved in the hydrogenationapparatus (a 1.33% solution) in 300 ml of dioxane and after the additionof 0.59 g of Raney Nickel it was pressured with hydrogen to 250 psi andallowed to react at ambient temperature for 8 hours. The catalyst wasremoved from the solution by filtration and the solution concentrated at20° C under 15 mm pressure, leaving a mixture of viscous oil containingsome yellow powder. Filtration of the viscous oil through a sinteredglass filter yielded 0.64 g of material which, when heated in a meltingpoint tube from 25° C to higher temperatures, did not melt up to 300° Cbut gave evidence of decomposition in the range of 175°-180° C.

EXAMPLE 7

In 200 ml of dry ethyl ether, (C₂ H₅)₂ O, was dissolved 1.93 g of4-nitrophthalic anhydride (˜ 1% solution) to which was added 0.10 g of5% palladium-on-charcoal and the mixture hydrogenated in a Paarhydrogenator at 39 psi at room temperature, until no more hydrogen wasconsumed. The catalyst was removed by filtration and the ether removedfrom the filtrate, maintained at 0° to -5° C, under vacuo, yielding ayellow crystalline solid mixed with non-crystalline viscous mass. When asmall sample of this mixture was placed on a metal plate heated to 245°C, the product melted and polymerized immediately to a tough polymer.Extraction of the crystalline-non-crystalline mixture with ether left0.46 g of polymer; concentration of the ether followed by cooling,yielded 0.86 g of 4-aminophthalic anhydride.

EXAMPLE 8

In 50 ml of freshly distilled deoxygenated N,N-dimethylformamide orN,N-dimethylacetamide containing 10 mg of 5% palladium-on-charcoalcatalyst, there was dissolved 5.8 g (0.03 mole) of 4-nitrophthalicanhydride (˜ a 10.6% solution) and the solution reduced with hydrogen at40 psi while maintained at 20° C, until no more hydrogen was consumed,requiring approximately 90 minutes. The solution was then filteredthrough a sintered glass filter, the filtrate cooled to about 0° C anddivided into two equal portions.

Portion A

One-half of the filtered solution was poured onto 200 g of very finelycrushed ice with the formation of a bright yellow precipitateaccompanied by the melting of the ice. The resulting water suspensionwas filtered rapidly, the precipitate isolated and dried, withoutheating, at 0.1 mm Hg pressure. The yield of dried product was 2.8 g(97%). It was not possible to record the infrared spectrum of the driedproduct as a KBr disc because of the reactivity of the 4-aminophthalicanhydride, since polymerization occurred when the product was groundwith KBr or when the mixture of the product and KBr were molded into adisc prior to measurement. However, its infrared spectrum could be takenas a smear in Nujol; the product showed the complete disappearance ofthe NO₂ bands present in the precursor 4-nitrophthalic anhydride at 1540cm⁻¹ and 1370 cm⁻¹, the retention of the anhydride bands at 1800 and1725 cm⁻¹ and the appearance of the NH₂ band at 3330 cm⁻¹.

Analysis: Calc'd for C₈ H₅ NO₃ ; NH₂ C₆ H₃ (CO)₂ O: C, 58.90; H, 3.09;N, 8.59. Found: C, 58.86; H, 3.08; N, 8.57.

Titration of a sample with 0.2 N NaOH established an equivalent weightof 163.1 compared to the theoretical value of 163 for product as dibasicacid anhydride.

The melting point of pure 4-aminophthalic anhydride could not bedetermined in the usual manner, either in a melting point tube or on aFisher-Johns apparatus. When heating of the specimen was started at roomtemperature, the sample was not observed to melt even when heated to300° C. When a sample was dropped on the preheated surface of theFisher-Johns disc, melting was not observed at temperatures below 207°C, but it was observed to melt at 208° C and at higher temperatures to aclear yellow liquid which increased rapidly in viscosity during thecourse of which its color changed from a yellow to an orange to a brownsolid film, which did not melt on further heating. This method ofdetermining the melting point is referred to hereafter as the melt-nomelt technique.

Portion B

The remaining half of the sample was concentrated at 15 mm Hg pressurewithout heat, leaving a viscous resinous mass. It was washed with a50:50 mixture of toluene-methanol and finally with methanol, and driedat 50° C at 15 mm Hg pressure for 24 hours. There was obtained 4.61 g(94%) of a polymer whose infrared spectrum showed bands for COOH in theregion of 3000 cm⁻¹, bands for CONH at 1660 cm⁻¹ ; weak bands for imideat 1765, 1725 and 725 cm⁻¹ ; weak bands for NH at 3330 cm⁻¹ ; and veryweak bands for anhydride at 1800 cm⁻¹. The inherent viscosity of theresin in dimethylacetamide at 25° C was 0.26 dl/g.

The potentiometric titration of a sample with 0.2 N NaOH establishedthat the polymer has one acid equivalent with an equivalent weight of259.3. This value corresponds with a polymer containing a ratio of threesegmers of hemiamic acid to two segmers of imide. ##STR10##

EXAMPLE 9

3-Nitrophthalic anhydride, 5.8 g, was 3: product isolated by the sameprocedure used to prepare its 4-isomer in Example 8. The yield was 94.6%and its melting point, determined by the melt-no melt technique used forthe 4-isomer was 192° C.

Analysis: Calc'd for C₈ H₅ NO₃ : C, 58.89; N, 3.09; N, 8.59. Found: C,58.85; H, 3.07; N, 8.60.

Its infrared spectrum was similar to that of its 4-isomer; it did notshow bands for NO₂ but did show bands for NH₂ in the 3330 cm⁻¹ regionand for --CO--O--CO-- in the 1800 and 1725 cm⁻¹ region.

EXAMPLE 10

A mixture of 3- and 4-nitrophthalic anhydride (5.8 g) was reduced andthe product isolated by the procedure of Example 7. The yield was 95.1%,melting point determined by the melt-no melt technique ranged from 165to 181° C. Its infrared spectrum showed the presence of bands for --NH₂at 3300 cm⁻¹ and for --CO--O--OC-- at 1800 and 1725 cm⁻¹ but no bandsfor NO₂.

Analysis: Calc'd for C₈ H₅ NO₃ : C, 58.89; H, 3.09; N, 8.59. Found: C,59.01; H, 3.10; N, 8.53.

EXAMPLE 11

In 50 ml of dry dioxane or tetrahydrofuran there was dissolved 1.93 g of4-nitrophthalic anhydride and 0.10 g of 5% palladium-on-charcoal addedand reduction with hydrogen at 56 psi performed at a temperaturemaintained at 15°-25° C in a shaking Paar apparatus until no morehydrogen was consumed. Then the solution was filtered to remove thecatalyst. A sample of the solution was cast on glass and heated in anoven at 50° C for 24 hours, to yield a film which adhered to the glass;on additional heating, at 100° C for 24 hours, a hard film was obtained.

The remainder of the solution was poured onto 200 g of finely crushedice with the formation of a yellow precipitate and partial melting ofthe ice. When most of the ice had melted, the precipitate was recoveredby filtration and dried in vacuo at room temperature to constant weightto yield 1.4 g of product. When a sample of the product was slowlyheated from room temperature to higher temperatures it did not melt, butwhen dropped on a metal block preheated to 210°-220° C, it melted to athin melt and polymerized rapidly to a hard polymer. Its infraredspectrum and its elemental analysis of percent C, 58.82; H, 3.07; N,8.56 confirms that this product was pure 4-aminophthalic anhydride andit is identical to the product prepared by the reduction performed indimethylacetamide. The monomer was soluble also in diethyl ether,acetone, methanol, ethanol and benzene.

EXAMPLE 12

This example characterizes the differences in the thermal behavior ofthe pure isomeric 3- and 4-aminophthalic anhydrides.

A. 3-Aminophthalic Anhydride.

1. Differential Thermal Analysis. The sample was heated at a rate of 3°C/minute under nitrogen. A sharp endotherm, which peaked at 190° C, wasobserved; this temperature compares with 192° C found for its meltingpoint by the melt-no melt technique reported hereinabove.

2. Thermogravimetric Analysis. The sample was heated at a rate of 3°C/minute under a nitrogen atmosphere at a flow of one standard liter perminute. Sublimation of the monomer was observed to begin atapproximately 125° C and to condense on the surface of the gas-exittube. Sublimation continued at a high rate to about 210° C, at whichpoint reaction occurred and continued to about 300° C without furthersublimation loss, after which water was eliminated to about 325° C; nofurther loss occurred on heating up to 500° C. In spite of the loss ofsublimation of approximately 45%, there remained, after heating to 500°C, a polymeric residue amounting to approximately 40% by weight of thestarting monomer.

B. 4-Aminophthalic Anhydride.

1. Differential Thermal Analysis

The sample was heated at a rate of 3° C/minute under nitrogen. A sharpendotherm which peaked at 207° C was observed; this temperature compareswith 207°-208° C found for its melting point by the melt-no melttechnique described hereinabove.

2. Thermogravimetric Analysis of Monomer

The sample was heated at a rate of 3° C/minute under a flow of nitrogenat one standard liter per minute. Sublimation of this 4- monomer did notoccur as in the case of its 3- isomer. An inflection in the curve becameevident in the range of 210°-241° C with a loss amounting to 11% at 315°C and 12% at 325° C, and 14% at 500° C. The calculated loss for theconversion of 4-aminophthalic anhydride to polyphthalimide is 11%. Thechar residue remaining at 500° C was 86%.

EXAMPLE 13 C. Mixed 3- and 4-Aminophthalic Anhydrides.

1. Differential Thermal Analysis

a. An equal molar mixture of 3- and 4-aminophthalic anhydride was usedin this test, following the same procedure as A.1 above. A broadexotherm was observed in the 170°-185° C region.

b. A sample of mixed anhydrides of Example 10 was treated by theprocedure of A.1. A broad exotherm in the region of 158°-184° C wasobserved which compared to 160°-181° C found by the melt-no melttechnique.

2. Thermogravimetric Analysis. The procedure described in A.1 was used.

a. An equal molar mixture of 3- and 4-aminophthalic anhydride was used.An inflection became evident at about 180° C with a loss amounting toabout 12% at 315° C. Little or no sublimation was observed.

b. The mixed anhydride of Example 10 gave little or no evidence ofsublimation and the behavior was similar to the synthetic mixture C.2.aabove.

EXAMPLE 14

One g of 4-aminophthalic anhydride was dissolved in 50 ml of absoluteethanol and heated at 50° C for 3 hours, following which the alcohol wasremoved at 5-10 mm Hg pressure without heating, leaving as a crystallinesolid the hemiester, m.p. 128° C; recrystallized from benzene, m.p. 131°C.

Analysis: Calc'd for C₁₀ H₁₁ NO₄ : C, 57.42; H, 5.30; N, 6.70. Found: C,57.23; H, 5.32; N, 6.75.

EXAMPLE 15

In 100 ml of deoxygenated absolute alcohol in a Paar bottle there wasdissolved 1.93 g of 4-nitrophthalic anhydride and 7 mg of 5%palladium-on-charcoal added. Then the apparatus was flushed withhydrogen and then pressured to 40.5 psi of hydrogen and the reductionallowed to continue for 12 hours at room temperature. Then the catalystwas removed by filtration and the filtrate concentrated at 10 mm Hgpressure at 25° C, leaving a residue which was recrystallized frombenzene, m.p. 131° C. The mixed melting point with the sample of Example14 was 130° C.

Analysis: Found: C, 57.13; H, 5.27; N, 6.71.

EXAMPLE 16

Solutions of pure 4-aminophthalic anhydride (AMPA) in dry, freshlydistilled dioxane were prepared at room temperature and then wereconcentrated to constant weight at 15 mm Hg pressure without heat. Inall cases, viscous resinous masses were obtained which were extractedwith benzene in which the monomer is soluble and the polymer insoluble.Concentration of the benzene extract to dryness gives values reported asthe monomer, the difference is reported as polymer (or oligomer). Thedata are shown in Table 1.

                  Table 1                                                         ______________________________________                                        4-AMPA      Dioxane  % Wt.    % Products                                      Solution                                                                             g        g        AMPA   Polymer                                                                              AMPA                                   ______________________________________                                        A      1        99        1     77     23                                     B      1        19        5     88     12                                     C      1        9        10     100    0                                      D      1        3        25     100    0                                      E      1        2        331/3  100    0                                      ______________________________________                                    

Similar results are obtained when solutions of AMPA are prepared in lowboiling solvents such as tetrahydrofurane and ethyl acetate, but in thehigher boiling solvents such as 1 and 5% by weight solutions in DMF andDMAC, respectively, evaporation of these solvents at room temperature,leaves only 2 and 1% unconverted 4-AMPA respectively.

EXAMPLE 17

Solutions of 3-aminophthalic anhydride are prepared in dioxane andconcentrated by the procedure of Example 6 and the data are given inTable 2.

                  Table 2                                                         ______________________________________                                               Weight %  % Products                                                   Solution of 3-AMPA   Polymer     Monomer                                      ______________________________________                                        A         1          70          30                                           B         5          82          18                                           C        10          98          2                                            D        25          100         0                                            E         331/3      100         0                                            ______________________________________                                    

The data indicate 3-AMPA is less reactive than 4-AMPA. In DMAC or DMF,the 1 and 5 weight percent solutions yield only 8 and 6% unreacted3-AMPA, respectively, on evaporation of the solvent compared to 2 and 1percent respectively for 4-AMPA.

EXAMPLE 18

Solutions of equal molar portions (1:1) of 4-AMPA and 3-AMPA areprepared in dioxane and concentrated by the procedure of Example 16 andthe data are given in Table 3.

                  Table 3                                                         ______________________________________                                               Weight % of                                                                             % Products                                                   Solution 1:1 anhydrides                                                                            Polymer     Monomer                                      ______________________________________                                        A         1           76         24                                           B         5           86         14                                           C        10          100         0                                            D        25          100         0                                            E        33          100         0                                            ______________________________________                                    

In the DMAC or DMF the 1 and 5% by weight solutions, respectively,leave, on evaporation of the solvent, 2.3 and 1.0% of unconvertedanhydrides.

EXAMPLE 19

The following example illustrates the use of aromatic diamines, NH₂--Ar--NH₂, or aromatic tetracarboxylic acid dianhydrides, ##STR11## asmodifiers in the polymerization of 4-AMPA. Solutions of 1.63 g of 4-AMPAwere prepared in DMAC at room temperature, and the modifier, when used,was then added to the amount of a molar ratio of 1 mole of modifier to100 moles of 4-AMPA and the polymerization conducted at 70° C for 8hours, and the inherent viscosity of the modified polymers was measuredas 0.5% solution in DMAC. The data are summarized in Table 4.

                  Table 4                                                         ______________________________________                                                Modifier* DMAC      Weight %                                                                              η inh                                 Solution                                                                              g         ml        Solid   Polymer                                   ______________________________________                                        A       none      5         25 approx.                                                                            0.310                                     B       PMA;0.022 5         25 approx.                                                                            0.481                                     C       MPD;0.011 5         25 approx.                                                                            0.442                                     D       PMA;0.022 3.37      33 approx.                                                                            0.600                                     E       MPD;0.011 3.37      33 approx.                                                                            0.561                                     ______________________________________                                         *PMA is pyromellitic anhydride                                                MPD is meta-phenylenediamine                                             

The inherent viscosity of the modified polymers is substantially higherthan the unmodified products. Corresponding improvements in inherentviscosity are obtained when instead of PMA there is used benzophenonetetracarboxylic acid dianhydride, or instead of MPD there is usedp-phenylenediamine, oxydianiline, methylene dianiline, sulfonyldianiline, H₂ NC₆ H₄ (OC₆ H₄)₂ NH₂, (H₂ NC₆ H₄ OC₆ H₄)₂ SO₂, and otherratios are used, such as in the range of 100 to 1000 moles of theaminophthalic anhydride per mole of diamine or dianhydride modifierrespectively.

EXAMPLE 20

This example illustrates the improvement in the inherent viscosity whenthe modifier is introduced into a hydrogenation system before the4-nitrophthalic anhydride is reduced to the 4-AMPA and polymerizationallowed to occur simultaneously with hydrogenation.

(a) To a solution in a hydrogenation vessel of 19.3 g of 4-nitrophthalicanhydride in 38.6 g of DMAC was added 0.50 g of 5% palladium-on-charcoalcatalyst and the mixture pressured with hydrogen to 75 psi heated to 60°C. Agitation was started and an exotherm raised the temperature to 70° Cwhere it was maintained during the course of 8 hours. The mixture wasthen cooled to 20°-25° C and filtered to remove the catalyst, yielding aviscous filtrate. The inherent viscosity was 0.41 measured as a 0.5% byweight solution in DMAC at 30° C.

(b) To a solution in a hydrogenation vessel of 19.3 g of 4-nitrophthalicanhydride and 0.22 g of pyromellitic anhydride in 38.6 g of DMAC wasadded 0.52 g of 5% palladium-on-charcoal catalyst and the mixturehydrogenated as in (a) above. The inherent viscosity was 0.63.

(c) To 38.6 g of DMAC there was dissolved 0.11 g of m-phenylenediamine,then 19.3 g of 4-nitrophthalic anhydride was added, followed by 0.51 gof 5% palladium-on-charcoal and the mixture hydrogenated as in (a)above. The inherent viscosity was 0.56. Substantially identical resultsare obtained if, instead of m-phenylenediamine, an equivalent amount ofmeta or para dinitrobenzene, 0.168 g, are used and reduced in situ.

Continuous films were obtained by the casting of these solutions onplates and drying at 130° C. The films from (b) and (c) were muchtougher than the film from (a).

(d) Procedures (a), (b) and (c) above were repeated in the presence of 7g of anhydrous magnesium sulfate, and solution having at least a 5%higher viscosity were obtained, 0.456, 0.701 and 0.622 dl/grespectively.

EXAMPLE 21

To establish that the modifiers of Example 19 function as couplingagents to increase the viscosity, Example 19 was repeated twice, usinginstead of MPD an equivalent amount of the precondensed product of theformula ##STR12## prepared by two independent methods, A and B, yieldingsubstantially identical results to those obtained in Example 19.

Method A.

Two moles of 4-nitrophthalic anhydride and one mole, as a 10% solution,of MPD, were dissolved in glacial acetic acid, then three moles ofacetic anhydride were added and the mixture heated at 100° C for 4hours, cooled to room temperature, poured onto crushed ice, filtered anddried, to yield

    O.sub.2 NC.sub.6 H.sub.3 (CO).sub.2 NC.sub.6 H.sub.4 N(OC).sub.2 C.sub.6 H.sub.3 NO.sub.2.

then 4.58 parts of the dinitro- compound were dissolved in 50 parts ofDMAC and reduced catalytically at 20° C by the procedure of Example 8.A, to give ##STR13## The elemental analysis percent C, 66.35; H, 3.5; N,14.07 and 0, 16.14 is in good agreement with the theoretical values.

Method B

To a solution of 10.8 g of MPD in 100 ml of m-cresol and 15 ml ofbenzene, there was added 32.6 g of 4-aminophthalic anhydride, and themixture reacted at reflux in a flask connected to a Dean-Stark trapuntil no more water was collected. Then the mixture was concentratedunder vacuum, washed with benzene and dried. Its elemental analysispercent, C, 66.33; H, 3.51; N, 14.04; 0, 16.08 is in very good agreementwith the compound of Method A.

EXAMPLE 22

The procedure of Example 21.B was used to react 1 mole of PMA and 10moleof AMPA by the benzene:m-cresol azeotropic process of Example 21.B togive

    O(OC).sub.2 C.sub.6 H.sub.3 N(OC).sub.2 C.sub.6 H.sub.2 (CO).sub.2 NC.sub.6 H.sub.3 (CO).sub.2 O, m.p. > 300° C.

analysis: Calc'd for C, 61.42; H, 1.57; N, 5.51. Found: C, 62.01; H,1.61; N, 5.46.

This precondensed product, when used in Example 19, instead of PMA, inan equivalent amount, yields polymers with properties substantiallyequivalent to those in which the PMA is not precondensed with AMPA.

EXAMPLE 23

An amine end-capped oligomeric polyimide (AEO-1) was prepared asfollows: In a m-cresol:benzene azeotropic apparatus of Example 21.B wasplaced BTCA* (11.2781 g, 0.035 mole), SDA-3,3** (10.8760 g, 0.0438mole), 80 ml of m-cresol and 10 ml of benzene. The ratio of BTCA-SDA-3,3is 4:5. The mixture was refluxed for 31/2 hours, during which time 1.3ml of water was collected. Then, the benzene was distilled off and thesolution was precipitated in methanol. The precipitated oligomer wasdigested three times in hot methanol and then vacuum dried at 70° C for24 hours to give 19.7498 g (95%) as a light-yellow solid which wassoluble in m-cresol, DMAC and sulfolane; it swelled in hot dioxane andmelted at 245°-280° C. A sample was vacuum-dried at 200° C and analyzed.

Analysis: Calc'd for C₁₂₈ H₆₈ N₁₀ O₃₀ S₅ : C, 64.42; H, 2.87; N, 5.87;O, 20.12; S, 6.72. Found: C, 63.71; H, 2.91; N, 5.75; O, ---; S, ---.

EXAMPLE 24

An amine end-capped oligomeric polyimide (AEO-2) was prepared asfollows: According to the procedure given in Example 23, there wasallowed to react BTCA (11.2781 g, 0.035 mole) and SDA-3,3 (9.7834 g,0.0394 mole) in 80 ml of m-cresol and 10 ml of benzene. The ratio ofSDA-3,3 to BTCA is 9:8. There was obtained the oligomer, AEO-2, as alight-yellow solid, 18.3 g (92.5%), which was soluble in m-cresol, DMACand sulfolane. It swelled in hot dioxane. On a Fisher-Johns apparatus itmelted partially at 255° C. The lowest temperature at which a samplewould melt completely when dropped onto the preheated block was 280° C.The analysis was performed on a small sample vacuum-dried at 200° C.

Analysis: Calc'd for C₂₄₄ H₁₂₄ N₁₈ O₅₈ S₉ : C, 64.77; H, 2.76; N, 5.57;O, 20.51; S, 6.38. Found: C, 63.54; H, 2.81; N, 5.45; O, ----; S, ----.

EXAMPLE 25

An amine end-capped oligomeric polyimide (AEO-3) was prepared asfollows: According to the procedure given in Example 23, BTCA (3.2223 g,0.01 mole) and DAPB-3,3* (3.6529 g, 0.0125 mole) were allowed to react.The ratio of BTCA to DAPB-3,3 is 4:5. There was obtained 6.1215 g (94%)of AEO-3 as a yellow powder which, on a Fisher-Johns melting pointapparatus melted in the range of 180°-200° C. It was soluble inm-cresol, DMAC, sulfolane and dioxane.

Analysis: Calc'd for C₁₅₈ H₈₈ N₁₀ O₃₀ : C, 72.81; H, 3.40; N, 5.37; O,18.42. Found: C, 72.72; H, 3.35; N, 4.77; O, ----.

EXAMPLE 26

An amine end-capped oligomeric polyimide (AEO-4) was prepared asfollows: According to the procedure given in Example 23, BTCA (3.2223 g,0.01 mole) and DAPB-3,3 (3.2887 g, 0.01125 mole) were allowed to react.The ratio of BTCA to DAPB-3,3 is 8:9. There was obtained AEO-4 as ayellow powder, 5.8708 g (95.4%). On a Fisher-Johns apparatus it began tomelt at 190° C but did not melt completely by 200° C. The lowesttemperature at which a sample melted when dropped onto a preheated blockwas 220° C. It was soluble in m-cresol, DMAC, sulfolane and dioxane.

Analysis: Calc'd for C₂₉₈ H₁₆₀ N₁₈ O₅₈ : C, 72.74; H, 3.28; N, 5.12; O,18.86. Found: C, 72.45; H, 3.31; N, 5.04; O, ----.

EXAMPLE 27

An anhydride end-capped oligomeric polyimide (ANEO-1) was prepared asfollows: According to the procedure of Example 23, there was allowed toreact BTCA (12,0837 g, 0.0375 mole) and SDA-3,3 (7.4493 g, 0.03 mole) in80 ml of m-cresol and 10 ml of benzene. The ratio of BTCA to SDA-3,3 is5:4. There was obtained the oligomer ANEO-1, as a light-yellow solid,16.9 g (92%) which was soluble in m-cresol, DMAC, DMF and sulfolane. Itsoftened at 240° C and melted from 245°-265° C.

Analysis: Calc'd for C₁₃₃ H₆₂ N₇ O₃₅ S₄ : C, 65.30; H, 2.56; N, 4.01; O,22.89; S, 5.24. Found: C, 63.90; H, 2.74; N, 4.70; O, ----; S, ----.

EXAMPLE 28

An anhydride end-capped oligomeric polyimide (ANEO-2) was prepared asfollows: According to the procedure of Example 23, there was allowed toreact BTCA (14.5003 g, 0.045 mole) and SDA-3,3 (9.9324 g, 0.04 mole) in90 ml of m-cresol and 20 ml of benzene. The ratio of BTCA to SDA-3,3 is9:8. There was obtained the oligomer, ANEO-2, as a light-yellow solid,21.4 g (95%) which was soluble in m-cresol, DMAC, DMF and sulfolane. Itmelted partially at 265° C. The lowest temperature at which a samplemelted completely when dropped onto a preheated block was 270° C.

Analysis: Calc'd for C₂₄₉ H₁₁₈ N₁₅ O₆₃ S₈ : C, 65.24; H, 2.60; N, 4.58;O, 11.99; S, 5.60. Found: C, 63.99; H, 2.73; N, 4.95; O, ----; S, ----.

EXAMPLE 29

An anhydride end-capped oligomeric polyimide (ANEO-3) was prepared asfollows: According to the procedure of Example 23, there was allowed toreact BTCA (4.0279 g, 0.0125 mole) and DAPB-3,3 (2.9223 g, 0.01 mole) in40 ml of m-cresol and 10 ml of benzene. The ratio of BTCA to DAPB-3,3 is5:4. There was obtained the oligomer ANEO-3 (5.7678 g, 80%) as alight-yellow powder which was soluble in m-cresol, DMAC, sulfolane anddioxane. On a Fisher-Johns melting point apparatus it melted over therange of 190°-205° C.

Analysis: Calc'd for C₁₅₇ H₇₈ N₈ O₃₅ : C, 71.52; H, 2.98; N, 4.25; O,21.24. Found: C, 71.41; H, 3.21; N, 4.46; O, ----.

EXAMPLE 30

An anhydride end-capped oligomeric polyimide (ANEO-4) was prepared asfollows: According to the procedure of Example 23 there was allowed toreact BTCA (3.6251 g, 0.01125 mole) and DAPB-3,3 (2.9223 g, 0.01 mole)in 40 ml of m-cresol and 10 ml of benzene. The ratio fo BTCA to DAPB-3,3is 9:9. There was obtained ANEO-4 (5,6071 g, 90%) as a light-yellowpowder which was soluble in m-cresol, DMAC, sulfolane and dioxane. On aFisher-Johns melting point apparatus it melted in the range of 185°-215°C.

Analysis: Calc'd for C₂₉₇ H₁₅₀ N₁₆ O₆₃ : C, 72.04; H, 3.09; N, 4.53; O,10.35. Found: C, 71.09; H, 3.22; N, 4.60; O, -----.

EXAMPLE 31

A 5 ml solution of Polymer B of Example 19 was mixed with a 5 mlsolution of Polymer C of Example 19 and allowed to stand at 25° C for 6hours. Coupling of the anhydride-terminated Polymer B with theamine-terminated Polymer C occurred, yielding a product, with aninherent viscosity of 0.583 dl/g in a 0.5% solution of DMAC.

Similarly, equivalent weights of Polymers D and E of Example 19 ,coupled when mixed, to increase the inherent viscosity of 0.674 dl/g.

EXAMPLE 32

Equal molar amounts of the amine-terminated pimer of Example 21A and ofthe anhydride-terminated dimer of 22A are first prepared as 30%solutions in DMAC and then mixed at 20° C to yield a blockpolyimide-polyamic acid, inherent viscosity 0.469 dl/g of the structure##STR14##

EXAMPLE 33

The oligomer AEO-1 of Example 23 (2.384 g) is dissolved in 10 ml of DMACand there is slowly added 1.63 g of AMPA, yielding a clear, viscoussolution of a polymer which can be generalized by the formula ##STR15##in which n' is 4 and n is 5.

In a similar way, instead of AEO-1, 4.28 g of AEO-2 of Example 24, 2.718g of AEO-3 of Example 25 and 4.915 g of AEO-4 of Example 26,respectively, are reacted with 1.63 g of AMPA. In the case of AEO-2 andAEO-4, the value of n' is 8. When 3.26 g of AMPA are used in the abovereactions, the value of n is 10.

EXAMPLE 34

The procedure of Example 26 is repeated and when the initialcondensation for AEO-4 is completed, 1.63 of AMPA is added and thereaction continued until no more water is collected in the water trapand there is obtained a completely cyclized block-polyimide instead ofthe hemiamic acid form shown in Example 33, which can be represented by##STR16##

The same procedure, using AEO-1, AEO-2 and AEO-3 instead of AEO-4,yields compounds conforming to the above structure with the appropriatevalues of n' and n; the value of n depending on the amount of AMPA used.

EXAMPLE 35

The oligomer ANEO-1 of Example 27 (2.444 g) is dissolved in 10 ml ofDMAC and 1.63 g of AMPA added and reacted as in Example 33, to yield aclear, viscous solution of polymer which can be generalized by theformula: ##STR17##

In a similar way, instead of ANEO-1, 4.585 g of ANEO-2 of Example 28;2.634 g of ANEO-3 of Example 29, and 4.946 g of ANEO-4 of Example 30,respectively, are reacted with 1.63. g of AMPA wth the appropriatevalues of n' and n, in which the value of n' is determined by the valuesin the ANEO-oligomers and n by the amount of AMPA reacted. When 3.26 gof AMPA is used, the value of n is 9.

EXAMPLE 36

The procedure of Example 30 is repeated and when the initialcondensation for ANEO-4 is completed, 1.63 g of AMPA is added and thereaction continued until no more water is collected and there wasobtained a completely cyclized block polyimide instead of the hemiamicacid form shown in Example 35, which can be written ##STR18##

Similarly, ANEO-1, ANEO-2 and ANEO-3 can be condensed with AMPA toproduce completely cyclized block polyimides.

EXAMPLE 37

Similar results are obtained when the procedure of Example 5A isrepeated a number of times using individually in place of the4-nitrophthalic anhydride equivalent amounts respectively of

3-nitro-4fluoro-phthalic anhydride,

4-nitro-5-chloro-phthalic anhydride,

3-nitro-5-bromo-phthalic anhydride,

3-nitro-4-butyl-phthalic anhydride.

As will be evident to those skilled in the art, various modifications ofthis invention can be made or followed in the light of the foregoingdisclosure without departing from the spirit or scope of this invention.

What is claimed is:
 1. The process of preparing a modified film-formingpolyamic acid which comprises reacting an aminophthalic acid of theformula H₂ NC₆ H_(n) Y_(3-n) (CO)₂ O wherein Y represents a halogenselected from the group consisting of F, Br and Cl, and n represents aninteger having a value of 0 to 3, in an inert organic solvent in whichthe polymer is soluble, at a temperature of about 10°-100° C with 0.5 to0.001 mole, per mole of the aminoanhydride, of a compound having theformula Ar(CO)₂ 0₂, in which Ar represents single, fused andheterocyclic aromatic rings, said heterocyclic rings being selected fromthe class consisting of pyridine quinoline and quinoxaline, and amultiplicity of such rings linked to each other directly or by --O--,--S--, --CO--, --SO₂ --, --CR₂ --, --NR--, ##STR19## linkages in which Rrepresents hydrogen or a hydrocarbon group containing one to twelvecarbon atoms, and the remaining positions in the Ar groups beingoccupied by hydrogen and a halogen selected from the class consisting ofF, Cl and Br.
 2. The process of claim 1 in which the modifier is O(CO)₂C₆ H₃ COC₆ H₃ (CO)₂
 0. 3. The process of claim 1 in which the modifieris O(CO)₂ C₆ H₂ (CO)₂
 0. 4. The film-forming polyamic acid whichconsists of the reaction product of an aminophthalic anhydride of theformula H₂ NC₆ H_(n) Y_(3-n) (CO)₂ O wherein Y represents a halogenselected from the class consisting of F, Br and Cl, and n represents aninteger having a value of 0 to 3, and about 0.1 to 0.001 mole of acompound having the formula Ar ((CO)₂ 0)₂ in which AR represents single,fused and heterocyclic aromatic rings, said heterocyclic rings beingselected from the class consisting of pyridine, quinoline andquinoxaline, and a multiplicity of such rings linked to each otherdirectly or by --O--, --S--, --CO--, SO₂ --, --CR₂ --, --NR--,--RC═CR--, ##STR20## linkages in which R represents hydrogen or ahydrocarbon group containing one to twelve carbon atoms, and theremaining positions in the Ar groups being occupied by hydrogen or ahalogen selected from the class consisting of F, Cl and Br
 5. Theprocess of claim 1 which comprises reacting at least 5 moles of saidaminophthalic anhydride with one mole of said compound having theformula Ar.
 6. The process of claim 5 in which said aminoanhydride is4-aminophthalic anhydride.
 7. The process of claim 5 in which saidaminoanhydride is 3-aminophthalic anhydride.
 8. The process of claim 5in which a mixture of 3- and 4-aminophthalic anhydride is used as theaminoanhydride.
 9. The process of claim 8 in which the relativeproportions 3- and 4-aminophthalic anhydrides corresponds to thequantity obtained by the reduction of the mixture of nitrophthalicanhydrides obtained by the nitration of phthalic anhydride.
 10. Theprocess of claim 5 in which 5 to 1000 moles of said aminophthalicanhydride are used per mole of said Ar ((CO)₂ O)₂ compound.
 11. Thedianhydride-terminated polymer produced by the process of claim
 10. 12.The process of claim 5 in which meta-cresol comprises a major portion ofthe solvent.
 13. The process of claim 5 in which an N,N-dimethylacylamide comprises a major portion of the solvent.
 14. The process ofclaim 5 in which N-methyl pyrrolidone comprises a major portion of thesolvent.
 15. The process of claim 5 in which an aromatic hydrocarbon ofthe formula C₆ H₃ R'₃ , wherein R' represents hydrogen or methyl,comprises a major portion of the solvent.